A method and a device for regulating the fuel/air ratio of a combustion process are described in German Published Patent Application No. 40 01 616.
Lambda gives the ratio of the actual air quantity participating in the combustion process to the air quantity which is required for a stoichiometric combustion of a certain fuel quantity. Exhaust gases of combustion processes are frequently passed through a catalytic converter in order to convert exhaust gas components such as nitrogen oxides (NOx), unburnt hydrocarbons (HC) and carbon monoxide (CO) to nitrogen, water and carbon dioxide. For instance, three-way catalysts are used for cleaning exhaust gas in motor vehicles.
An optimum efficiency of the conversion, which is characterized in response to specified charges of NOx, HC and CO in the catalytic converter by a minimum of NOx, HC and CO after the catalytic converter, requires a precise setting of a desired fuel/air ratio for the combustion process. This may also include the most precise possible setting of a desired behavior over time, such as periodic fluctuation of lambda about an average setpoint value.
With regard to the optimized conversion of catalytic conversion systems in motor vehicles, conventionally an exhaust gas probe downstream from the catalytic converter ensures the converter's optimum operation with respect to pollutants. Nernst probes are primarily used for this purpose. A Nernst probe is understood to be an oxygen-sensitive exhaust gas sensor, which has a characteristic curve, plotted against the mixture composition that is in thermodynamic equilibrium within the range of the stoichiometric mixture composition, which has a steep transition between a low (approximately 100 mV) and a high (approximately 900 mV) signal level.
Conventional methods may be summarized by a generic term two-step control. The concept of two-step control includes a regulation in which the actual value of the probe signal, which corresponds to an actual oxygen concentration in the exhaust gas, and thus to an actual lambda value, is compared to a setpoint value, and at which value, depending on the sign of the deviation, an enrichment or a leaning of the fuel/air ratio is generated. This regulation is distinguished by the fact that only the sign but not the absolute value is processed by a regulating algorithm.
Conceptually, two-step controls, and this applies to two-step control probes, are used upstream and downstream from a catalytic converter. These methods have in common that they react to the above-mentioned steep transition of the probe signal by an abrupt change in the control variable, such as an injection pulse width. The abrupt advance is followed by an approximately static change in the control variable, which as a pattern over time that corresponds to a ramp (i.e., it is linear). The lambda value of the optimum pollutant conversion in the catalytic converter does not correspond exactly to the lambda value of the steep change in the Nernst probe signal. In order nevertheless to be able to set the optimum value for the catalytic converter, using the Nernst probe, depending on the direction of the sign change, one may use a different and thus non-symmetrical step change height, a ramp following a step change and non-symmetrical with respect to the step change direction, or a predetermined delay time between a probe signal change and a control variable change. Thereby the average value of the pattern over time of the control variable is shifted such that the catalytic converter is operated at an optimum operating point. This lies mostly somewhat on the rich operation side, since, performing in this manner, one avoids, in particular, a safety distance from the lean operation side, which is more critical with regard to undesired NOx emissions. This manner of two-step regulation is frequently performed on a basis of a signal of an exhaust gas probe situated upstream from the catalytic converter. The oscillation in the oxygen content of the exhaust gas occurring during a step-change ramp regulation is averaged by the catalytic converter, provided it is functional. This averaging occurs because the catalytic converter, during the half wave of the oscillation during excess oxygen, stores the excess oxygen from the exhaust gas, and gives off the stored oxygen during the half wave of the oscillation having the lack of oxygen. An exhaust gas probe situated downstream from the (sufficiently large) catalytic converter in this case registers the average value of the oscillation. Since the preconnected catalytic converter protects the downstream probe from excessive temperature fluctuations, and also promotes the setting of the thermodynamic equilibrium of the exhaust components, the signal of the downstream probe is less influenced by temperature influences and cross sensitivities of the exhaust gas probe. In this context, cross sensitivity is understood to mean an undesired shifting of the probe characteristic curve plotted against the oxygen content in the exhaust gas in the presence of other exhaust gas components. Therefore, the downstream probe measures more accurately and may be used to guide the upstream probe. If, for example, the upstream probe regulates to an incorrect setpoint value because of the shifting of a characteristic curve, this is recognized via the signal of the downstream exhaust gas probe, and the setpoint value for the regulating circuit of the upstream probe is appropriately corrected.
Also conventional are so-called stepless methods. These do not utilize the steep change of a Nernst probe signal, but rather the comparatively linear pattern of the pump current as a function of the lambda value in the case of a wide range lambda probe. These methods use not only the sign, but also the absolute value of the deviation of an actual value from a setpoint value. Here too, one should observe that the catalytic converter is operated using a slightly rich mixture. Since smaller probe signal changes are used in these methods, the cross sensitivities, temperature sensitivities and aging deterioration-specific shifting of pollutant dependencies have a comparatively strong effect.
A further group of methods is based on an optimized filling strategy of the catalytic converter. The methods of this group strike a balance of the charged components and attempt to adjust a faulty balance before it is to be measured by the probe situated downstream from a certain catalyst volume. The Nernst probe is operated in the rich branch of its curve, and just equalizes a false balance zero point. German Published Patent Application No. 40 01 616 illustrates such a method for regulating the fuel/air ratio of a combustion process which is alternatingly operated with excess air and deficiency of air. A catalyst volume in the exhaust gas of the combustion process stores oxygen during excess of oxygen in the exhaust gas, and releases it again during oxygen deficiency. In this method the oxygen charge taking place into the catalyst volume during an excess of air, and the oxygen discharges from the catalyst volume during air deficiency are determined with the aid of a Nernst probe situated upstream from the catalytic converter, and the fuel/air ratio is regulated such that the sum of the oxygen charges and the oxygen discharges during a predetermined interval takes on a predetermined value.
It has been shown that future legal requirements, such as the SULEV requirements (super ultra low emission vehicle) in the United States of America will require further improvements of regulating strategies, with regard to optimized catalytic converter operation in conjunction with further increased robustness and regulating speed.
This requirement may be fulfilled by the method described in German Published Patent Application No. 40 01 616 on the basis that the combustion process is operated respectively at least as long at excess oxygen or oxygen deficiency until it appears at an oxygen-sensitive Nernst probe downstream from the catalyst volume. In a modification of the method, in one exemplary embodiment of the present invention, no exhaust gas probe is required upstream from the catalytic converter. In a further exemplary embodiment, a wide range lambda probe is used upstream from the catalytic converter, instead of the Nernst probe.